Brain Research 887 2000 276–284 www.elsevier.com locate bres
Research report
Valproate prevents the induction of sensitization to methylphenidate ritalin in rats
a,b a
a,b a
b ,
Pamela Yang , Anitra Beasley , Kary Eckermann
, Alan Swann , Nachum Dafny
a
Department of Psychiatry and Behavioral Sciences , The University of Texas Medical School at Houston, PO Box 20708, Houston, TX 77225, USA
b
Department of Neurobiology and Anatomy , The University of Texas Medical School at Houston, PO Box 20708, Houston, TX 77225, USA
Accepted 19 September 2000
Abstract
Repetitive exposure to methylphenidate MPD elicits sensitization to its locomotor effects. Drugs that affect the GABA system may modify adaptations to drug exposure. Therefore, we have examined the effect of sodium valproate, which enhances GABA function, on
the development of sensitization to MPD using an automated, computerized animal activity monitoring system to record each rat’s motor activities for 15 consecutive days. Rats were recorded before and after saline injection Days 1–2 to provide baseline activity. Animals
were then randomly assigned to the following three groups that received: 1 2.5 mg kg MPD s.c. for six consecutive days Days 3–8, 2 a single dose of valproate 50 mg kg; i.p. 1 h prior to the first Day 3 of six daily doses of MPD 2.5 mg kg; s.c., or 3 five daily
doses of valproate 50 mg kg, i.p. 1 h prior to MPD 2.5 mg kg, s.c. on Days 4–8. There was no drug treatment during the next 5 days Days 9–13. All rats were then re-challenged with MPD 2.5 mg kg, s.c. on Day 14. Group 2 rats were also re-challenged with 50
mg kg valproate followed by 2.5 mg kg MPD 1 h later on Day 15. Administration of MPD alone produced a sensitized response. Multiple valproate injections prevented the induction of MPD-elicited sensitization in all four motor indices, while a single valproate
injection prevented the induction of MPD-elicited sensitization in two of four motor indices studied. In conclusion, a single injection 50 mg kg valproate given prior to any MPD treatment partially blocked the induction of MPD sensitization while repeated injections of
valproate co-administered with MPD treatment completely prevented this effect.
2000 Elsevier Science B.V. All rights reserved.
Theme : Neurotransmitters, modulators, transporters, and receptors
Topic : Behavioral pharmacology
Keywords : Sensitization; Locomotor activity; Psychostimulants; GABA; Dopamine
1. Introduction Increased locomotor activities and stereotypic movements
incited by dopaminergic stimulation can be blocked by GABA agonists at doses where these agonists have no
Repeated administrations of psychomotor stimulants direct effects [7,28]. Clonazepam, a potent GABA-benzo-
such as amphetamine, cocaine [20,21], and methylpheni- diazepine agonist, was reported to prevent the development
date [11] have been shown to cause behavioral sensitiza- of stimulant-induced sensitization [16]. Sodium valproate,
tion. Behavioral sensitization is characterized by an in- an anticonvulsant drug used in treating schizophrenia and
creased locomotor and or stereotypic behavior resulted mania [24], can enhance GABA activity and has effects
from repetitive treatment over time to a given dose of a similar to that of benzodiazepine derivatives in conditional
stimulant [27]. Systemic administration of GABA agonists avoidance response studies [26]. The present study was
reduces dopamine turnover in the mesolimbic system [2]. carried out to determine whether enhancement of GABA
activity from a single pretreatment of valproate prior to any MPD administration or multiple doses of valproate
Corresponding author. Tel.: 11-713-500-5616; fax: 11-713-500-
given 1 h before MPD administration would prevent the
0621. E-mail address
: Nachum.Dafnyuth.tmc.edu N. Dafny.
induction of locomotor sensitization to MPD.
0006-8993 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 2 9 9 6 - 6
P . Yang et al. Brain Research 887 2000 276 –284
277
2. Materials and methods which grouped them into several different locomotor
indices. Animal activity was recorded continuously for 15 2.1. Subjects
days. The following four locomotor indices were analyzed: 1
Male Sprague–Dawley rats N528; Harlan, Houston, horizontal activity HA, which measured the overall
TX, weighing from 180 to 230 g, were housed in groups motor activity in the lowest tier of the testing cages and
of four in Plexiglas cages inside the experimental room on assessed the overall amount of motor activity, 2 total
a 12-h light dark schedule light on at 07:00 h. The distance TD, which recorded the specific motor behavior
experimental room was maintained at an ambient tempera- of forward movement, 3 vertical activity VA which
ture of 21628C and at a relative humidity of 37–42. counted the specific instances of rearing, and 4 number
After a minimum of 48 h of habituation, each rat was of stereotypes NOS which determined the stereotypic
randomly placed inside a test home cage where its behaviors, measured by repeated interruption of the same
locomotor activity recording began in all groups after 24 h beam [10].
of acclimation to the home cages and continued for 15 days. Rats were supplied with food pellets and water ad
2.4. Control and treatment groups libitum.
There were three experimental groups: Group 1 N512 2.2. Drugs
— after baseline and saline, animals treated with 6 days of MPD followed by five washout days and MPD re-chal-
Both sodium valproate VAL; Research Biochemicals lenge on Day 14 Table 1; Group 2 N58 — similar to
International, Natick, MA and methylphenidate hydro- Group 1 but, in addition, animals were given a single
chloride MPD; Sigma Chemicals, St Louis, MO were administration of valproate 50 mg kg on Day 3 and Day
dissolved in 0.9 saline. Valproate was injected intraperi- 15 1 h prior to MPD; and Group 3 N58 — multiple
toneally i.p., while MPD was injected subcutaneously administrations of valproate 50 mg kg was given on
s.c.. All injections were of equal volume 0.8 ml and Days 4–8 1 h prior to each dose of MPD Table 1. The
given between 12:00 h and 14:00 h. The selected valproate following is the general protocol for these groups: Day 1
dose was 50 mg kg and MPD dose was 2.5 mg kg. These — baseline activity; Day 2 — saline injection 0.9, i.p.;
dosages were selected based on previous dose–response Day 3 — Groups 1 and 3 were treated with MPD, Group 2
studies [8,10,11]. It was shown in preliminary studies that was given valproate 1 h prior to MPD; Days 4–8 —
the dose of valproate chosen had no motor effect of its Groups 1 and 2 were treated with daily MPD, Group 3 was
own [8]. given valproate 1 h prior to daily MPD on Days 4–8; Days
9–13 — washout for all three groups; Day 14 — MPD 2.3. Apparatus
was given to all three groups; and Day 15 — valproate 1 h prior to MPD was given to Group 2.
A computerized animal activity monitoring CAAM, AccuScan Instruments, Inc., Columbus, OH system was
2.5. Data analysis used to record locomotor activities continuously through-
out the 15 experimental days [10]. The activity chambers Variability in baseline activity among animals was
consisted of clear acrylic open field boxes 40.5340.53 observed; therefore, the best comparison for the drug
31.5 cm with two levels tiers, each consists of 16 infrared effects is to have each animal served as its own control.
beams. The first and second levels of the tiers were placed The drug effect for each motor index was determined by
6 and 12.5 cm from the cage floor, respectively. The subtracting the activity score of the 2-h after saline
CAAM recorded any interruption of each of the beams at a injection Day 2 from that of the 2 h after MPD injection
frequency of 100 Hz. The interruption of any beam was to eliminate any handling and injection effects [10–12].
counted. Total counts were compiled and downloaded Thus, the activity of saline injection served as baseline and
every 10 min into the OASIS data collection program, the absolute change in activity from saline represented
Table 1 Treatment schedule
Day 1
2 3
4–8 9–13
14 15
Group 1 N512 H
B S
MPD 14.00 MPD 14:00
WO MPD 14:00
WO Group 2 N58
H B
S VAL 13:00,
S 13:00, WO
S 13:00, VAL 13:00,
MPD 14:00 MPD 14:00
MPD 14:00 MPD 14:00
Group 3 N58 H
B S
S 13:00, VAL 13:00;
WO S 13:00,
WO MPD 14:00
MPD 14:00 MPD 14:00
H5habituation; B5baseline; S50.9 saline i.p.; MPD52.5 mg kg methylphenidate s.c.; WO5washout; VAL550 mg kg sodium valproate i.p..
278 P
effects of the drugs. For Groups 1 and 3, the presence of In the line graph Fig. 1, each 10-min sample on Day 3
sensitization in each treatment group was determined by was compared to the same sample on Day 14 for the initial
comparing results of Days 4 to 8 and 14 with that of MPD 120 min following MPD administration. With respect to
challenge on Day 3 first day of MPD administration in HA and VA, the first seven samples i.e., 70 min after
¨ naıve animals within that group. For Group 2, the
MPD injection significantly differed in Days 3 and 14 presence of sensitization was determined by comparing
P,0.01. For TD, five of the initial samples of Day 14 results of Days 5 to 8 and 14 to 15 with that of Day 4
after MPD treatment differed significantly from Day 3. second day of MPD administration, without valproate.
The differences in Day 3 and 14 were strongest in NOS Results were subjected to a within-group repeated measure
with the first seven samples 70 min after injection; P, analysis of variance ANOVA, two levels: treatment day
0.001. Thus, MPD induced activities were greatest within and 10 min sample. The significant differences in in-
the first 70 min after drug injection. dividual 10-min samples were determined with post-hoc
Fischer’s LSD method. Significance for all of the above 3.2. Single administration of valproate
comparisons was set at P,0.05, P,0.01, or P, 0.001. Values are presented as the mean6S.E.M. for each
The effect of a single dose of valproate given 1 h prior 10 min after injection or for the sum of 2-h sample. In
to MPD injection on Day 3 is summarized in Fig. 2. A addition, the difference in the magnitude of motor re-
single injection of valproate given prior to any MPD sponses was also determined by calculating the ratio of the
treatment prevented the induction of MPD sensitization for absolute change in activity between MPD challenge Day
TD and NOS as expressed in the first 2 h after MPD 3 for Groups 1 and 3; Day 4 for Group 2 and MPD
injection. However, there were significant differences on re-challenge Day 14. Results were analyzed using re-
Day 7 Fig. 2 bar graph and 14 from Day 3 with respect to peated measure analysis of variance one-way ANOVA:
HA P,0.05 and VA P,0.05 for Day 7; P,0.01 for treatment day, and any significant difference between the
Day 14; thus, a single valproate injection did not prevent groups was determined with the Dunnett test. Significance
the induction of MPD-sensitization for these indices. The for this comparison was set at P,0.05.
effects of the second injection of valproate 1 h prior to MPD Day 15 also differed significantly from the effects
of the first valproate injection 1 h prior to MPD Day 3;
3. Results P,0.05 for VA, while there were no significant differ-
